Hanging idle
 

I've gone around and around with this, and cannot find what is causing my hanging idle.

I just replaced the distributor, so it's not that.

Car has a Quadrajet rebuilt by Quadrajet power. I just put the Progression Distributor on it, loaded what I think is a pretty conservative curve, set the idle mixture as best I'm able, and took it for a drive. It sounds good, no pinging or bogs. But it still does something that it has done since the restoration was finished - once you drive it around and it's good and warm, the idle will hang up about 150-200rpm above the set point. When you turn off the car and start it up, it will go right back where it belongs. Choke is properly adjusted as best I can tell, the throttle lever appears to be seated against the stop. There is a yellow throttle return spring installed.

This cannot be replicated by holding the throttle open or blipping it in the driveway. It always comes back to the set point under those conditions.

Almost seems vacuum related since it resets to the set idle when the car is shut down and restarted.

Anyone have any ideas here?

How are the motor mounts?
Yes I see that the car has been restored, but they can rip out anytime if they are defective and then in gear the motor rolls over and takes the slack out of the throttle cable.

I've seen Q-Jets where the throttle blades don't return to proper position because they're hanging-up on a gasket.

Worn throttle shaft bushings are known for screwing with idle speed problems, too.

When my Holley tri-power had erratic idle speed, the throttle blade screws were loose. I got lucky--they didn't actually come out, just loose enough that the blades moved around on the shaft.

I’m just taking a stab in the dark but what about the secondaries? Is the linkage attached and adjusted so they fully close when the primaries close? If not, could it be possible after driving/opening the secondaries they hang up slightly due to the vacuum signal through the carb? Shut the car off and spring tension is enough to pull them shut? Or even the secondary air flaps?

After this occurs, can you shut the engine down then re-fire it within seconds and it will return to its lower idle?

I’ll check it out, thanks!

Mounts were new less than 1000 miles ago, I sure don’t see the shaker moving around much. “In gear” doesn’t rally apply, it’s a manual car.

Yes, when it happens and it’s shut down, immediate restart will idle at the right speed. I haven’t messed with the carb other than adjusting choke and idle mixture since I got it from Quadrajet Power, so I would hope all the linkages are good, but I don’t know enough to really tell. I’ll have to get in there and poke around. Thanks!

The cure will be with the throttle plates and shafts assuming the linkage back to the secondaries has adequate clearance to keep from holding the secondaries open.

I get a LOT of carbs in here completely rebuild or fully restored with that issue. It's usually the huge secondary throttle plates not correctly centered and indexed so they are fully seating in the bores. Remove the base plate, gently loosen the screws, tap the shaft back and forth to center it and tap the throttle plates into the base plate with a screwdriver handle. Very gently tighten the screws, hold it up to a light and make sure they are fully seated before torquing them on down. Not uncommon to find the shaft twisted slightly and need some "tweaking"......



Too tight on the primary bushings will cause the same issue, or plates not correctly centered, so spend some time there as well.

Thank you Cliff - I have your book, can you point me to a page that might walk me through this operation in some measure? Thanks!

maybe this will help
https://www.youtube.com/watch?v=XpKexG5Y5NM

Check your PCV Valve, pull it out of the grommet, put your thumb over the open end and see if your idle changes. If so change your PCV Valve making sure it is a factory type. I've seen many PCV Valves bought from auto parts stores cause problems.

Of course the idle will "change" when you block-off a controlled, calibrated air leak into the fuel/air mixture.

Doesn't mean the PCV valve is defective. It means that there's less air getting into the manifold, but with the same amount of fuel being delivered.

Resets?
 

Resets when you cut the engine off and start it back up.

I'd be checking initial timing at set idle and when idle is higher.

How much difference have you got in idle RPM and the RPM that distributor starts the timing curve?

Clay

I now have a Progression distributor that controls the timing electronically, so weights and vacuum canister aren't involved. Timing is progressive based on the intervals in the timing table. The issue is unchanged since I changed from the conventional HEI.

verify timing when it is high with a timing gun vs after restart, compare against table values...

spring return not enough?

I've confirmed that the secondary butterfly is sticking. Reached under the air cleaner and manually pushed the lever for the secondary butterflies, and the idle dropped. So do I need a new gasket for when I put the baseplate back together, or are they reusable to some extent? The carb was completely rebuilt about 2 years ago, probably has under 200 miles on it.

Had your same exact issue with my 1979 Trans Am when I first got it. Can you actually hear or feel a higher RPM when it hangs, does your oil pressure go up to coincide?

My idle would hang up around the same amount, especially after coming from a stop from a long cruise in 4th gear. I did everything - heavy weight advance springs, replaced all vacuum lines, even the intake gasket; popped my carb open more times than I can count, had it sent to QuadrajetPower for primary rebushiny and secondary linkage recalibrating, made sure throttle plates were returning to where they should and that the secondaries were shut at idle, made sure it wasn't my AC idle solenoid.... everything.

Ended up concluding it was just a whacky tach lol. When I took an RPM-reading timing light to it after a long drive, even after it "hanged" a bit on the tach, the RPM still read the same as what I had set it to. So now, I just look at my oil pressure gauge now to verify I'm at my proper idle. 750ish RPMs for me is exactly 40PSI right in the middle when I'm warmed up. Hasn't bothered me since.

Not coming off fast idle.

Gasket is probably reusable. That’s what Cliff told me when I asked him about mine before I rebuilt my Qjet and he was right it was fine to reuse. And my gasket was probably 8+ years old.

Definitely not just the tach. Confirmed it by physically moving the secondary shaft. Everything else coincides with the RPM increase.

Definitely off the fast idle cam. Only idles high after driving hard enough to get into the secondaries.